I intend to mainly use the iPhone for Whatsapp or ebuddy. How much battery life can you expect on the iPhone 4 and 4S?

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I guess i'm the first to ACTUALLY answer your question.
To my experience, on wifi, if you just use mainly whatsapp, sms on wifi on 50% brightness. At the end of the day you'll still have around ~75% left. They don't consume much battery.
But if you do more things like intense gaming, whatsapp, 3G web surfing, videos, expect to have ~45-50% left by the end of the day.

In the end, those are all just estimates according to my experience. There are many other factors that chip in when it comes to battery. eg. if someone uses their screen at 100% brightness, then tough luck. It's gonna die pretty quickly.

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